1. Field of the Invention
The present invention relates to a regulated high-voltage power supply which comprises a tuned-collector type oscillation circuit to obtain stabilized high-voltage output by controlling the base current of an oscillation transistor of the oscillation circuit.
2. Description of the Prior Art
FIG. 2 is an electric circuit diagram showing a regulated high-voltage power supply proposed in Japanese Patent Application No. 6182/1983 (Japanese Patent Laying-Open Gazette No. 132776/1984 published on July 30, 1984). Referring to FIG. 2, the high-voltage power supply 10 comprises a tuned-collector/grounded-emitter type oscillation circuit 3 including a regenerative capacitor C2, an oscillation transistor TR4 and the like and a control circuit 2 for controlling the base current of the oscillation transistor TR4. The high-voltage power supply 10 further comprises a resistance circuit 4 for supplying the base current to the oscillation transistor TR4 in response to the output from the control circuit 2 and a protection circuit 5 having a protective transistor TR3 which partially receives the output from the resistance circuit 4 for attenuating the output of the control circuit 2 when the output is short-circuited in the area of a load RL. The resistance circuit 4 includes a first resistance circuit part 41 having high resistance value and a second resistance circuit part 42 including a zener diode ZD. The first resistance circuit part 41 is formed by a plurality of series-connected resistors R3 and R4, the junction P2 between which is connected to the base of the protective transistor TR3 of the protection circuit 5. A negative feedback control circuit 6 is provided for negative feedback of the output from a detecting resistor R9 which is connected to a high-voltage output side as shown by arrows A, thereby to control the operation of the control circuit 2. Reference numeral 7 indicates an output circuit for supplying high-voltage output to the load RL in response to the output from the oscillation circuit 3. Symbol V.sub.in indicates an input terminal of a DC power supply and symbol V.sub.out indicates an output terminal. As hereinabove described, symbol RL indicates the load connected to the output terminal V.sub.out .
The oscillation circuit 3 further includes an oscillation stabilizing resistor R7, positive feedback winding L3, a tuning capacitor C3 and primary (low-voltage) winding L1 of a high-voltage transformer T. The first resistance circuit part 41 of the resistance circuit 4 is not primarily adapted to supply the base current to the oscillation transistor TR4, but to form a short-circuit detecting circuit for detecting whether or not the high-voltage output is short-circuited. Therefore, the resistors R3 and R4 are set at high resistance value. The second resistance circuit part 42 of the resistance circuit 4 forms a main base-current network for mainly supplying the said base current, and the resistor R5 thereof is set at low resistance value. The negative feedback control circuit 6 comprises a comparison amplifier AMP. A negative-phase input terminal (-) of the comparison amplifier AMP receives the output from the detecting resistor R9 as shown by arrows A while a positive-phase input terminal (+) thereof receives the output from a reference power supply E1. The output circuit 7 includes secondary (high-voltage) winding L2 of the high-voltage transformer T, a rectifier diode D1, a smoothing capacitor C4 and a spark discharge preventing resistor R8. In addition, a delay capacitor C5 is provided between the junction P2 of the resistors R3 and R4 forming the first resistance circuit part 41 and a grounded point P4 for delaying the rise time of operation voltage Vb of the protective transistor TR3 at the junction P2 with respect to the rise time of output voltage Va from the control circuit 2.
When the power supply circuit is activated in such structure, the negative-phase input terminal (-) of the comparison amplifier AMP in the negative feedback control circuit 6 is at a low input level. Therefore, the output of the negative feedback control circuit 6 rises toward a high level, whereby the output voltage Va at an output point P1 of the control circuit 2 is increased. The rise speed of the operation voltage Vb at the junction P2 is slower than that of the output voltage Va, through the function of the aforementioned delay capacitor C5.
Therefore, the output voltage Va rises to a level for making the zener diode ZD conductive before the operation voltage Vb at the junction P2 is increased to make the protective transistor TR3 conductive. Consequently, the base of the oscillation transistor TR4 is supplied with the base current, whereby the oscillation circuit 3 rapidly enters an oscillating state. Thus, when the oscillation circuit 3 enters a normal oscillating state, the oscillation transistor TR4 is continuously supplied with the base current since the output voltage Va at the output point of the control circuit 2 is set to be higher than the zener voltage of the zener diode ZD. Further, the operation voltage Vb at the junction P2 is set to be lower than the driving voltage for the protective transistor TR3, so that the protective transistor TR3 is not operated.
When the high-voltage output is short-circuited, the oscillation is so attenuated that voltage Vc at a junction P3 between the regenerative capacitor C2 and the oscillation stabilizing resistor R7 is increased, whereby the operation voltage Vb at the junction P2 is also increased. As the result, the protective transistor TR3 is made conductive to decrease the output voltage Va of the control circuit 2. The decreased output voltage Va is set to be lower than the zener voltage Vz of the zener diode ZD. Therefore, the zener diode ZD is made non-conductive and no substantial base current is supplied to the oscillation transistor TR4, whereby the oscillation circuit 3 stops the oscillation. In addition, although a fine or small base current still flows through the first resistance circuit part 41 to the oscillation transistor TR4, the oscillation will not be developed because of the short-circuiting of the high-voltage output. Thus, the power supply 10 is protected against short-circuiting of the high-voltage output.
When the short-circuiting of the high-voltage output is released, the oscillation circuit 3 starts fine oscillation by the fine base current flowing through the first resistance circuit part 41, whereby the voltage Vc of the regenerative capacitor C2 starts lowering and the protective transistor TR3 is made non-conductive. Since the rise speed of the operation voltage Vb at the junction P2 is slower than that of the output voltage Va of the control circuit 2 as hereinabove described, the protective transistor TR3 is not operated thereafter and the zener diode ZD is made conductive. Consequently, the oscillation transistor TR4 is supplied with the required base current, whereby the oscillation circuit 3 smoothly re-starts oscillation, so that the high-voltage power supply 10 may be restored to a state supplying a prescribed high-voltage output.
A description will now be presented of, e.g., an electrostatic copying machine, in which the high-voltage output of the high-voltage power supply 10 is applied to a charging device, and discharged to electrify the surface of a photosensitive drum.
When, for example, the copy papers are jammed in the said copying machine, corona discharge in a normal loaded state may be shifted to spark discharge. When such spark discharge takes place, the protective transistor TR3 is made conductive in a state similar to the aforementioned state in which the output is short-circuited in the area of the load RL and the current supply to the oscillation circuit 3 is substantially cut off, whereby the oscillation is stopped and the high-voltage output discharge is also stopped.
However, so long as spark discharge continues, the output is intermittently short-circuited, and moreover the protective transistor TR3 enters a non-conductive state from a conductive state in a short period of time since the amount of the storage charge in the delay capacitor C5 is small. Therefore, the protective transistor TR3, which was conductive, is made non-conductive immediately, so that current supply to the oscillation circuit 3 is started whereby the oscillation circuit 3 starts oscillation, to again cause spark discharge. Such intermittent operation is repeated within a short period of time, whereby discharge energy per unit time is increased to cause paper burning, leading to danger of fire. Further, such intense discharge may break or damage circuits etc. around the high-voltage power supply 10.